Molecular interactions between bacterial symbionts and their hosts

Symbiotic bacteria are important in animal hosts, but have been largely overlooked as they have proved difficult to culture in the laboratory. Approaches such as comparative genomics and real-time PCR have provided insights into the molecular mechanisms that underpin symbiont-host interactions. Studies on the heritable symbionts of insects have yielded valuable information about how bacteria infect host cells, avoid immune responses, and manipulate host physiology. Furthermore, some symbionts use many of the same mechanisms as pathogens to infect hosts and evade immune responses. Here we discuss what is currently known about the interactions between bacterial symbionts and their hosts.     

昆虫共生菌的次级代谢产物研究进展

微生物与昆虫的共生是一种普遍现象,昆虫种类繁多,与昆虫共生的微生物也多种多样。昆虫共生菌是活性次生代谢产物的重要来源。本文对自2008年以来已报道的177个昆虫共生菌的次级代谢产物进行了统计和分析,结果表明:61.6%的化合物为新天然产物(生物碱类新化合物最多),其中,约75%的新化合物来源于昆虫共生真菌,25%来源于细菌;醌酮类化合物是昆虫共生菌源天然产物的主要结构类型,占23.2%;47.5%的化合物具有显著的抗肿瘤、抗菌、除草和抗氧化等生物活性,且化合物中的主要活性类型是抗菌和抗肿瘤活性,活性范围覆盖面最广的结构类型是生物碱类。以上结果表明昆虫共生菌的次级代谢产物是先导性化合物的重要来源且具有丰富的生物活性类型。本文以天然产物的结构分类为切入点,结合其研究菌株来源、生物活性等进行综述,旨在为充分挖掘昆虫共生菌次级代谢产物提供重要参考。     

昆虫共生微生物在病虫害和疾病控制上的应用前景

昆虫与微生物之间的互利共生关系是自然界中一种常见的互作形式。昆虫的种类丰富多样并且在自然界中分布广泛,在一定程度上得益于共生微生物的帮助。随着生物技术的不断发展,越来越多的共生微生物和互利共生模式得以发现并深入研究。微生物不仅能够为昆虫的生长发育提供营养,还能合成很多生物活性物质、调节宿主的免疫、对抗捕食者和抵御病原微生物感染,成为宿主昆虫健康和适应的守护者。鉴于共生微生物与昆虫生理生态的密切联系,以及昆虫对人类经济与健康的重要影响,利用共生微生物对昆虫及虫媒病进行生物控制已经成为一个热点研究方向,并展现了良好的应用前景。本文对昆虫共生微生物的多样性、生物学功能、与宿主相互作用机制及其在病虫害和虫媒病防治中的研究进展进行综述和展望。     

培菌昆虫相关放线菌的次级代谢产物研究进展

昆虫共生微生物是一种特殊的微生物资源,其中放线菌在昆虫肠道、体表和巢穴中广泛分布。近年来,人们从培菌昆虫来源的放线菌中分离得到多种新型化合物,可以选择性抑制菌圃的致病真菌,部分还对植物致病真菌、昆虫致病真菌、人类病原菌和癌细胞有抑制活性。因此,研究培菌昆虫相关微生物不仅有助于了解宿主与微生物的共生机制,还能发掘新的活性物质,用于生物农药、生物医药的开发。本文对培菌昆虫来源放线菌次级代谢产物的研究进展进行了综述。     

Bacterial symbionts in insects: balancing life and death

Arthropods, particularly insects, form successful long-term symbioses with endosymbiotic bacteria. The associations between insects and endosymbionts are remarkably stable; many stretch back several hundred million years in evolutionary time. With the exception, perhaps, of the filarial nematodes no other group of metazoans shows such a proclivility for their intracellular symbionts. The identification and classification of bacterial symbionts and hosts has grown rapidly over the last two decades and these relationships form a continuum from classical mutualism to parasitism. Complete genomes have been sequenced for many of these bacteria and some of their hosts. Now more intractable questions regarding endosymbiosis are being addressed. Investigations on the role of the host immune system in the maintenance of symbiosis, the nature of bacteriophages and transposable elements found in the genomes of many bacterial symbionts, and the molecular mechanisms involved in establishing reproductive phenotypes such as parthenogenesis, male killing, cytoplasmic incompatibility and feminization have been recently reported. This review will focus on the impact of the secondary endosymbionts Wolbachia, Cardinium, and Spiroplasma on host fitness and immunity and will revisit the question of whether these bacteria are friend or foe from an insect’s point of view.     

Side-stepping secondary symbionts: widespread horizontal transfer across and beyond the Aphidoidea

To elucidate the co-evolutionary relationships between phloem-feeding insects and their secondary, or facultative, bacterial symbionts, we explore the distributions of three such microbes--provisionally named the R-type (or PASS, or S-sym), T-type (or PABS), and U-type--across a number of aphid and psyllid hosts through the use of diagnostic molecular screening techniques and DNA sequencing. Although typically maternally transmitted, phylogenetic and pairwise divergence analyses reveal that these bacteria have been independently acquired by a variety of unrelated insect hosts, indicating that horizontal transfer has helped to shape their distributions. Based on the high genetic similarity between symbionts in different hosts, we argue that transfer events have occurred recently on an evolutionary timescale. In several instances, however, closely related symbionts associate with related hosts, suggesting that horizontal transfer between distant relatives may be rarer than transmission between close relatives. Our findings on the prevalence of these symbionts within many aphid taxa, along with published observations concerning their effects on host fitness, imply a significant role of facultative symbiosis in aphid ecology and evolution.     

Bacterial Endosymbiosis in a Chordate Host: Long-Term Co-Evolution and Conservation of Secondary Metabolism

Intracellular symbiosis is known to be widespread in insects, but there are few described examples in other types of host. These symbionts carry out useful activities such as synthesizing nutrients and conferring resistance against adverse events such as parasitism. Such symbionts persist through host speciation events, being passed down through vertical transmission. Due to various evolutionary forces, symbionts go through a process of genome reduction, eventually resulting in tiny genomes where only those genes essential to immediate survival and those beneficial to the host remain. In the marine environment, invertebrates such as tunicates are known to harbor complex microbiomes implicated in the production of natural products that are toxic and probably serve a defensive function. Here, we show that the intracellular symbiont Candidatus Endolissoclinum faulkneri is a long-standing symbiont of the tunicate Lissoclinum patella, that has persisted through cryptic speciation of the host. In contrast to the known examples of insect symbionts, which tend to be either relatively recent or ancient relationships, the genome of Ca. E. faulkneri has a very low coding density but very few recognizable pseudogenes. The almost complete degradation of intergenic regions and stable gene inventory of extant strains of Ca. E. faulkneri show that further degradation and deletion is happening very slowly. This is a novel stage of genome reduction and provides insight into how tiny genomes are formed. The ptz pathway, which produces the defensive patellazoles, is shown to date to before the divergence of Ca. E. faulkneri strains, reinforcing its importance in this symbiotic relationship. Lastly, as in insects we show that stable symbionts can be lost, as we describe an L. patella animal where Ca. E. faulkneri is displaced by a likely intracellular pathogen. Our results suggest that intracellular symbionts may be an important source of ecologically significant natural products in animals.     

Ecological characteristics of insects that affect symbiotic relationships with mites

Parasites and pathogens that begin as symbionts, i.e., organisms living together in the same habitat, are some of the most promising drivers of species evolution. Because insects are highly diverse and important as ecosystem service agents and because mites can exert large effects on insect populations (capable of killing at least juveniles), insect–mite interactions have been analyzed from various perspectives, including evolutionary, ecological and pest‐management perspectives. Here, I review and examine insect–mite symbiotic associations to develop hypotheses concerning the factors that maintain and develop their relationships. Previous studies have hypothesized that insect sociality and mite richness and specificity affect insect–mite interactions. I found that both solitary and social insects, including parasocial and subsocial insects, harbor numbers of symbionts including species‐specific ones but few dangerous mite symbionts in their nests or habitats under natural conditions. Nest size or the amount of food resources in a nest may affect mite richness. On the basis of this review, I hypothesize that the insect characteristics relevant for mite symbiotic hosting are sharing the same habitat with mites and living in a nutrient‐rich habitat. I also suggest that many cases of species‐specific symbiosis began with phoresy. To test these hypotheses, phylogenetic information on mites living with insect groups and quantitative analysis to characterize each insect–mite relationship are necessary.     

Insect Gut Symbiont Susceptibility to Host Antimicrobial Peptides Caused by Alteration of the Bacterial Cell Envelope

The molecular characterization of symbionts is pivotal for understanding the cross-talk between symbionts and hosts. In addition to valuable knowledge obtained from symbiont genomic studies, the biochemical characterization of symbionts is important to fully understand symbiotic interactions. The bean bug (Riptortus pedestris) has been recognized as a useful experimental insect gut symbiosis model system because of its cultivatable Burkholderia symbionts. This system is greatly advantageous because it allows the acquisition of a large quantity of homogeneous symbionts from the host midgut. Using these naïve gut symbionts, it is possible to directly compare in vivo symbiotic cells with in vitro cultured cells using biochemical approaches. With the goal of understanding molecular changes that occur in Burkholderia cells as they adapt to the Riptortus gut environment, we first elucidated that symbiotic Burkholderia cells are highly susceptible to purified Riptortus antimicrobial peptides. In search of the mechanisms of the increased immunosusceptibility of symbionts, we found striking differences in cell envelope structures between cultured and symbiotic Burkholderia cells. The bacterial lipopolysaccharide O antigen was absent from symbiotic cells examined by gel electrophoretic and mass spectrometric analyses, and their membranes were more sensitive to detergent lysis. These changes in the cell envelope were responsible for the increased susceptibility of the Burkholderia symbionts to host innate immunity. Our results suggest that the symbiotic interactions between the Riptortus host and Burkholderia gut symbionts induce bacterial cell envelope changes to achieve successful gut symbiosis.     

Chemosynthetic endosymbioses: adaptations to oxic-anoxic interfaces

Chemosynthetic endosymbioses occur ubiquitously at oxic-anoxic interfaces in marine environments. In these mutualisms, bacteria living directly within the cell of a eukaryotic host oxidize reduced chemicals (sulfur or methane), fueling their own energetic and biosynthetic needs, in addition to those of their host. In habitats such as deep-sea hydrothermal vents, chemosynthetic symbioses dominate the biomass, contributing substantially to primary production. Although these symbionts have yet to be cultured, physiological, biochemical and molecular approaches have provided insights into symbiont genetics and metabolism, as well as into symbiont-host interactions, adaptations and ecology. Recent studies of endosymbiont biology are reviewed, with emphasis on a conceptual model of thioautotrophic metabolism and studies linking symbiont physiology with the geochemical environment. We also discuss current and future research directions, focusing on the use of genome analyses to reveal mechanisms that initiate and sustain the symbiont-host interaction.     

Manifold aspects of specificity in a nematode–bacterium mutualism

Coevolution in mutualistic symbiosis can yield, because the interacting partners share common interests, to coadaptation: hosts perform better when associated with symbionts of their own locality than with others coming from more distant places. However, as the two partners of a symbiosis might also experience conflicts over part of their life cycle, coadaptation might not occur for all life‐history traits. We investigated this issue in symbiotic systems where nematodes (Steinernema) and bacteria (Xenorhabdus) reproduce in insects they have both contributed to kill. Newborn infective juveniles (IJs) that carry bacteria in their intestine then disperse from the insect cadaver in search of a new host to infect. We ran experiments where nematodes coinfect insects with bacteria that differ from their native symbiont. In both Steinernema carpocapsae/Xenorhabdus nematophila and Steinernema feltiae/Xenorhabdus bovienii symbioses, we detected an overall specificity which favours the hypothesis of a fine‐tuned co‐adaptation process. However, we also found that the life‐history traits involved in specificity strongly differ between the two model systems: when associated with strains that differ too much from their native symbionts, S. carpocapsae has low parasitic success, whereas S. feltiae has low survival in dispersal stage.     

昆虫共生菌的垂直传播

共生菌普遍存在于昆虫体内,它们能够为宿主昆虫提供生长发育所必需的氨基酸、固醇类等营养物质,还能提高昆虫适应高温、寄生虫、病毒等不利环境因素的能力,昆虫则为共生菌提供稳定的生存环境和营养物质,昆虫与共生菌相互依存。多数情况下,共生菌通过垂直传播在宿主代次间进行传播,即共生菌由母代传递给子代。结合最近几年相关研究,本文综述了不同昆虫共生菌的垂直传播模式。除极少数肠道共生菌通过污染卵壳被宿主幼虫取食得以垂直传播外,垂直传播的共生菌多为经卵传播。根据侵染时期的不同,共生菌经卵传播模式多数可分为以下4种：侵染宿主昆虫幼虫中的生殖干细胞、侵染宿主昆虫年轻雌成虫中的生殖干细胞、侵染宿主昆虫雌成虫中的成熟卵母细胞以及侵染宿主昆虫囊胚期胚胎。其中,有些共生菌是以共生菌菌胞整体侵染的方式进入到宿主卵巢。另外,少数肠道共生菌也通过卵巢进行垂直传播,此类共生菌先侵染卵巢侧输卵管并在侧输卵管聚集,待卵排放至侧输卵管时再进入到卵中。在文中,我们也探讨了昆虫共生菌垂直传播过程中的细胞机制和免疫机制,包括共生菌避开宿主免疫反应、共生菌通过内吞作用进入卵巢以及不同共生菌间的协同作用等。     

Streptomyces as symbionts: an emerging and widespread theme?

Streptomyces bacteria are ubiquitous in soil, conferring the characteristic earthy smell, and they have an important ecological role in the turnover of organic material. More recently, a new picture has begun to emerge in which streptomycetes are not in all cases simply free-living soil bacteria but have also evolved to live in symbiosis with plants, fungi and animals. Furthermore, much of the chemical diversity of secondary metabolites produced by Streptomyces species has most likely evolved as a direct result of their interactions with other organisms. Here we review what is currently known about the role of streptomycetes as symbionts with fungi, plants and animals. These interactions can be parasitic, as is the case for scab-causing streptomycetes, which infect plants, and the Streptomyces species Streptomyces somaliensis and Streptomyces sudanensis that infect humans. However, in most cases they are beneficial and growth promoting, as is the case with many insects, plants and marine animals that use streptomycete-produced antibiotics to protect themselves against infection. This is an exciting and newly emerging field of research that will become increasingly important as the search for new antibiotics switches to unusual and under-explored environments.     

Microbial symbionts of marine invertebrates: opportunities for microbial biotechnology

Marine invertebrates are sources of a diverse array of bioactive metabolites with great potential for development as drugs and research tools. In many cases, microorganisms are known or suspected to be the biosynthetic source of marine invertebrate natural products. The application of molecular microbiology to the study of these relationships will contribute to basic biological knowledge and facilitate biotechnological development of these valuable resources. The bryostatin-producing bryozoan B. neritina and its specific symbiont "Candidatus Endobugula sertula" constitute one promising model system. Another fertile subject for investigation is the listhistid sponges that contain numerous bioactive metabolites, some of which originate from bacterial symbionts.     

Metabolic complementarity and genomics of the dual bacterial symbiosis of sharpshooters

Mutualistic intracellular symbiosis between bacteria and insects is a widespread phenomenon that has contributed to the global success of insects. The symbionts, by provisioning nutrients lacking from diets, allow various insects to occupy or dominate ecological niches that might otherwise be unavailable. One such insect is the glassy-winged sharpshooter (Homalodisca coagulata), which feeds on xylem fluid, a diet exceptionally poor in organic nutrients. Phylogenetic studies based on rRNA have shown two types of bacterial symbionts to be coevolving with sharpshooters: the gamma-proteobacterium Baumannia cicadellinicola and the Bacteroidetes species Sulcia muelleri. We report here the sequencing and analysis of the 686,192–base pair genome of B. cicadellinicola and approximately 150 kilobase pairs of the small genome of S. muelleri, both isolated from H. coagulata. Our study, which to our knowledge is the first genomic analysis of an obligate symbiosis involving multiple partners, suggests striking complementarity in the biosynthetic capabilities of the two symbionts: B. cicadellinicola devotes a substantial portion of its genome to the biosynthesis of vitamins and cofactors required by animals and lacks most amino acid biosynthetic pathways, whereas S. muelleri apparently produces most or all of the essential amino acids needed by its host. This finding, along with other results of our genome analysis, suggests the existence of metabolic codependency among the two unrelated endosymbionts and their insect host. This dual symbiosis provides a model case for studying correlated genome evolution and genome reduction involving multiple organisms in an intimate, obligate mutualistic relationship. In addition, our analysis provides insight for the first time into the differences in symbionts between insects (e.g., aphids) that feed on phloem versus those like H. coagulata that feed on xylem. Finally, the genomes of these two symbionts provide potential targets for controlling plant pathogens such as Xylella fastidiosa, a major agroeconomic problem, for which H. coagulata and other sharpshooters serve as vectors of transmission.     

Candidatus Sodalis melophagi sp. nov.: phylogenetically independent comparative model to the tsetse fly symbiont Sodalis glossinidius

Bacteria of the genus Sodalis live in symbiosis with various groups of insects. The best known member of this group, a secondary symbiont of tsetse flies Sodalis glossinidius, has become one of the most important models in investigating establishment and evolution of insect-bacteria symbiosis. It represents a bacterium in the early/intermediate state of the transition towards symbiosis, which allows for exploring such interesting topics as: usage of secretory systems for entering the host cell, tempo of the genome modification, and metabolic interaction with a coexisting primary symbiont. In this study, we describe a new Sodalis species which could provide a useful comparative model to the tsetse symbiont. It lives in association with Melophagus ovinus, an insect related to tsetse flies, and resembles S. glossinidius in several important traits. Similar to S. glossinidius, it cohabits the host with another symbiotic bacterium, the bacteriome-harbored primary symbiont of the genus Arsenophonus. As a typical secondary symbiont, Candidatus Sodalis melophagi infects various host tissues, including bacteriome. We provide basic morphological and molecular characteristics of the symbiont and show that these traits also correspond to the early/intermediate state of the evolution towards symbiosis. Particularly, we demonstrate the ability of the bacterium to live in insect cell culture as well as in cell-free medium. We also provide basic characteristics of type three secretion system and using three reference sequences (16 S rDNA, groEL and spaPQR region) we show that the bacterium branched within the genus Sodalis, but originated independently of the two previously described symbionts of hippoboscoids. We propose the name Candidatus Sodalis melophagi for this new bacterium.     

Marine actinomycetes: An ongoing source of novel bioactive metabolites

Actinomycetes are virtually unlimited sources of novel compounds with many therapeutic applications and hold a prominent position due to their diversity and proven ability to produce novel bioactive compounds. There are more than 22,000 known microbial secondary metabolites, 70% of which are produced by actinomycetes, 20% from fungi, 7% from Bacillus spp. and 1–2% by other bacteria. Among the actinomycetes, streptomycetes group are considered economically important because out of the approximately more than 10,000 known antibiotics, 50–55% are produced by this genus. The ecological role of actinomycetes in the marine ecosystem is largely neglected and various assumptions meant there was little incentive to isolate marine strains for search and discovery of new drugs. The search for and discovery of rare and new actinomycetes is of significant interest to drug discovery due to a growing need for the development of new and potent therapeutic agents. Modern molecular technologies are adding strength to the target-directed search for detection and isolation of bioactive actinomycetes, and continued development of improved cultivation methods and molecular technologies for accessing the marine environment promises to provide access to this significant new source of chemical diversity with novel/rare actinomycetes including new species of previously reported actinomycetes.     

Insect life history and the evolution of bacterial mutualism

Bacterial symbiosis has played a fundamental role in the evolution of eukaryotes. However, we still know little about how cooperative relationships with bacteria originate, and why they form in some host species but not others. Facultative symbionts that are beneficial, but not essential, provide unique insights into these processes. We use data from over a hundred aphid species to test if host life history is associated with the presence of facultative symbionts. We find that aphid species that have mutualistic associations with ants that protect them from natural enemies are less likely to carry symbionts that provide similar benefits. We also find one symbiont species occurs more frequently in unrelated aphid species that specialise on certain plant genera. In addition, aphid species that attack multiple plants often carry different symbiont complements. Our findings provide evidence of the ecological conditions that facilitate stable, mutually beneficial relationships between microbes and eukaryotic hosts.     

Molecular cross-talk between sponge host and associated microbes

Marine organisms especially those that live sessile, as sponges, are well known to have specific relationships with a great variety of microorganisms including bacteria and fungi. As most simple metazoan phylum, the Porifera, which emerged first during the transition from the non-Metazoa to the Metazoa from the common ancestor, comprise wide arrays of recognition molecules, both for Gram-negative bacteria and for Gram-positive bacteria as well as for fungi. They react specifically with effector molecules to inhibit or kill the invading microorganisms. The elicitation and the subsequent effector reactions of the sponges towards these microbes are outlined. However, besides of the elimination of bacteria and fungi, some of those taxa are kept as symbionts of the sponges, allowing them, for example, to accumulate the essential element manganese or to synthesize carotinoids. The sponges produce low-molecular-weight bioactive compounds, secondary metabolites, to eliminate the microorganisms. In addition, they are armed with cationic antimicrobial peptides allowing them to defend against invasive microorganisms and, in parallel, to kill or repel also metazoan invaders. The broad range of chemically and functionally different compounds qualifies the Porifera as the most important animal phylum to be exploited as a source for the isolation of new potential drugs. First molecular biological strategies have been outlined to obtain those compounds in a sustainable way, by producing them recombinantly.     

Localization and transmission route of Coriobacterium glomerans, the endosymbiont of pyrrhocorid bugs

Endosymbiotic gut bacteria play an essential role in the nutrition of many insects. Most of the nutritional interactions investigated so far involve gammaproteobacterial symbionts, whereas other groups have received comparatively little attention. Here, we report on the localization and the transmission route of the specific actinobacterial symbiont Coriobacterium glomerans from the gut of the red firebug, Pyrrhocoris apterus (Hemiptera: Pyrrhocoridae ). The symbionts were detected by diagnostic PCRs and FISH in the midgut section M3, in the rectum and in feces of the bugs as well as in the hemolymph of some females. Furthermore, adult female bugs apply the symbionts to the surface of the eggs during oviposition, from where they are later taken up by the hatchlings. Surface sterilization of egg clutches generated aposymbiotic insects and thereby confirmed the vertical transmission route via the egg surface. However, symbionts were readily acquired horizontally when the nymphs were reared in the presence of symbiont-containing eggshells, feces, or adult bugs. Using diagnostic PCRs and partial sequencing of the 16S rRNA gene, closely related bacterial symbionts were detected in the cotton stainer bug Dysdercus fasciatus (Hemiptera: Pyrrhocoridae ), suggesting that the symbiosis with Actinobacteria may be widespread among pyrrhocorid bugs.